1. Field of the Invention
The present invention relates to a light irradiation apparatus, a crystallization apparatus, a crystallization method and a device. In particular, the present invention relates to a crystallization apparatus and a crystallization method which generate a crystallized semiconductor film by irradiating a non-single-crystal semiconductor film with a laser light having a predetermined light intensity distribution.
2. Description of the Related Art
A thin film transistor (TFT) used for a switching element or the like which selects a display pixel in, e.g., a liquid crystal display (LCD) has been conventionally formed by using amorphous silicon or polysilicon.
Polysilicon has a higher mobility of electrons or holes than that of amorphous silicon. Therefore, when a transistor is formed by using polysilicon, the switching speed and hence a display response speed become higher than those in the case of forming the same by using amorphous silicon. Further, a peripheral LSI can comprise a thin-film transistor. Furthermore, there is an advantage of reducing the design margin of any other component. Moreover, when peripheral circuits such as a driver circuit or a DAC are incorporated in a display, these peripheral circuits can be operated at higher speed.
Since polysilicon comprises an aggregation of crystal grains, when, e.g., a TFT transistor is formed, a crystal grain boundary or boundaries are present in its channel region, this crystal grain boundary serves as a barrier, and a mobility of electrons or holes is reduced as compared with that of single-crystal silicon. Additionally, each of many thin-film transistors formed by using polysilicon has a different number of crystal grain boundaries formed in its channel portion, and this difference becomes irregularities, resulting in a problem of unevenness in display in case of a liquid crystal display. Thus, there has been recently proposed a crystallization method which generates crystallized silicon with a large particle (grain) size enabling at least one channel region to be formed in order to improve the mobility of electrons or holes and reduce irregularities in number of crystal grain boundaries in a channel portion.
As this type of crystallization method, there has been conventionally known a phase-control excimer laser annealing (ELA) method which forms a crystallized semiconductor film by irradiating a phase shifter approximated in parallel to a non-single-crystal semiconductor film (a polycrystal semiconductor film or an amorphous semiconductor film) with an excimer laser. Details of the phase-control ELA method are disclosed in, e.g., Journal of The Surface Science Society of Japan, Vol. 21, No. 5, pp. 278-287, 2000.
In this phase-control ELA method, a light intensity distribution having an inverse peak pattern (a pattern in which a light intensity is minimum at the center and the light intensity is suddenly increased toward the periphery (lateral sides)) in which the light intensity at a point corresponding to a phase shift portion (line or point) of a phase shifter is lower than that in the periphery is generated, and a non-single-crystal semiconductor film is irradiated with a light beam having this light intensity distribution with the inverse peak shape. As a result, a temperature gradient is generated in a molten area in accordance with the light intensity distribution in an irradiation target area, a crystal nucleus is formed at a part which is solidified first or a part which is not molten in accordance with a point where the light intensity is minimum, and a crystal grows from the crystal nucleus in a lateral direction toward the periphery (which will be referred to as a “lateral growth” hereinafter), thereby generating a single-crystal grain with a large particle size.
Further, “Arrays of Large Si Grains Grown at Room Temperature for x-Si TFTs” by M. Jyumonji, et al., SID 04 Digest, pp. 434, 2004 discloses that positioning of a growth start point of a crystal is performed by irradiating a non-single-crystal semiconductor film with a light beam having a light intensity distribution with an inverse peak shape generated by a phase step of a phase shifter. Further, this reference also describes a technique which optimally adjusts a light intensity at a bottom peak (an inverse peak point) in a light intensity distribution with an inverse peak shape by appropriately setting a phase difference (a phase quantity) of the phase step.
As described above, according to the latter reference, a bottom peak value in a light intensity distribution with an inverse peak shape is determined by a phase difference of the phase step. Specifically, as shown in FIG. 16A, when a phase shifter 191 which has a phase step 191a having a phase difference of 180° is used, a light intensity distribution with an inverse peak shape formed at a focus plane (an image formation surface) of an image formation optical system is symmetrical with respect to a line 191b corresponding to the phase step indicated by a broken line as shown in FIG. 16C, and a light intensity at its bottom peak 192 is substantially zero. Furthermore, a light intensity distribution with an inverse peak shape formed on a defocus plane vertically slightly moved from the focus plane of the image formation optical system is likewise substantially symmetrical as shown in FIGS. 16B and 16D, and a light intensity at its bottom peak 192 slightly increases from zero but is very small.
In this manner, when the phase shifter having the phase difference of 180°, since the symmetry of the light intensity distribution is maintained without being dependent on a defocus direction, a deep depth of focus can be realized. However, since the light intensity at the bottom peak is very small, the light intensity becomes not greater than a crystal growth start intensity (a light intensity with which the crystal growth starts) in an area having a certain degree of superficial content in the vicinity of the bottom peak. As a result, in the vicinity of the bottom peak, an amorphous state remains unchanged or, even if an irradiation target is molten, it remains in a polysilicon or fine crystal state, and hence an area ratio of a target part which becomes a crystal with a large particle size (i.e., a filling ratio of a crystal grain) cannot be increased. Here, the filling ratio is a ratio of a crystallized area with respect to an irradiation surface when a light having a light intensity distribution with an inverse peak shape is applied.
Conversely, as shown in FIG. 17A, when a phase shifter 193 which has a phase step 193a having a phase difference of 60° is used, a light intensity distribution with an inverse peak shape formed on a focus plane of an image formation optical system becomes substantially symmetrical with respect to a virtual line 193b corresponding to the phase step 193a and indicated by a broken line as shown in FIG. 17C and a light intensity at its bottom peak 192 becomes high to some extent. On the contrary, at a defocus position vertically slightly moved from the focus position of the image formation optical system, as shown in FIGS. 17B and 17D, the symmetry of the light intensity distribution with the inverse peak shape to be formed largely collapses, and a position of its bottom peak is shifted (moved). It is to be noted that a board thickness deviation which can be a factor of defocusing unavoidably exists in a processed substrate which is held at the focus position of the image formation optical system.
In this manner, when the phase shifter having the phase difference of 60° is used, the light intensity at the bottom peak becomes high and approximates the crystal growth start intensity as compared with the case of using the phase shifter having the phase difference of 180°, thereby enlarging a crystallized area. However, in case of the phase shifter having the phase difference of 60°, the symmetry of the light intensity distribution largely collapses at the defocus position vertically moved from the focus position. Further, since the way the symmetry collapses is inverted being dependent on the defocus direction between the light intensity distribution shown in FIG. 17B and the light intensity distribution depicted in FIG. 17D, a depth of focus becomes shallow (narrow).
Furthermore, since a position of the bottom peak is shifted in a plane by defocusing, a position of a crystal grain to be generated is also shifted from a desired position, which becomes a problem when forming a circuit by using this crystal grain. That is, when a crystal grain is not formed at a desired position, a boundary or boundaries of crystal grains exist in a channel portion of a transistor, thereby deteriorating characteristics of the transistor. The term “phase” pertaining to the present invention will be defined as follows, with reference to FIG. 18.
Consider the wavefront of an incident plane wave, which lies immediately behind a phase shifter. That part of the wavefront, which shifts in the propagation direction of light, is defined as “phase-advancing” side region. That part of the wavefront, which shifts toward the light source, is defined as “phase-delaying” side region. As FIG. 18 shows, the phase shifter has a protruding or thick part and a depressing or thin part on one surface. These parts border each other at a stepped portion. The protracting or projecting part is at the phase-advancing side region, and the depressing or receding part is at the phase-delaying side region.
This definition of phase can be applied also to other phase shifters that have neither a projecting part or a receding part. The phase may be controlled by using a fine pattern having lower resolution than the focusing optical system used. In this case, it suffices to apply the same definition of phase to the wavefront formed in the imaging field. For any phase shifter, the phase has a positive value if it advances. For example, +90° means a phase advance, and −90° a phase delay.
Moreover, when the phase shifter having the phase difference of 60° is used, as shown in FIGS. 17B and 17D, a peak intensity provided on one side of peaks provided on both sides of a bottom peak is raised an greatly increased in a light intensity distribution with an inverse peak shape at a defocus position. As a result, there is an inconvenience that a semiconductor film is destroyed due to ablation at this peak position on one side. It is to be noted that the side where the peak intensity is increased is a phase delay side (the left side of the phase shifter 193 in the figure) of a phase step at a defocus position apart from an image formation optical system, and it is a phase advance side (the right side of the phase shifter 193 in the figure) of the phase step at a defocus position close to the image formation optical system.